1. Field of the Invention: This invention relates in general to earth boring tools and in particular to improved tungsten carbide inserts and methods for their manufacture to improve retention in interferringly sized holes and to decrease the frequency of fracture of the inserts.
2. Description of the Prior Art: Earth boring tools include those which have tungsten carbide inserts interferringly retained in mating holes, with protruding ends formed as rock cutting, crushing, chipping or abrading elements. A typical insert is manufactured of sintered tungsten carbide, a composition of mono and/or ditungsten carbide cemented with a binder selected from the iron group, consisting of cobalt, nickel or iron. Cobalt ranging from about 10% to 16%, balance tungsten carbide, is presently the most common binder. The exact composition depends upon the usage intended for the tool and its inserts.
Most of the prior art inserts have round cylindrical surfaces dimensioned to be larger than their mating drilled and reamed holes by a nominal value of about 0. 002 inch. For the purpose of improving the surface finish and condition, the inserts are commonly "bright tumbled", meaning that they are tumbled against each other until their surfaces are smoothed to a condition that appears bright or shiny. Also, the inserts have in the past been tumbled in an abrasive medium such as aluminum oxide to produce a "mat" finish, which is relatively smooth but dull in comparison with a "bright" finish.
The smooth, bright and mat finishes were thought to be advantageous in minimizing, during insertion or pressing, the possibility of shearing the metal forming the surface of the interferring hole. Shearing of the retaining surface decreases the pressure exerted against an insert, and may cause loss of the insert during earth boring operations.
Frank E. Stebley in U.S. Pat. No. 3,581,835, issued June 1, 1971, disclosed a generally polygon shaped insert abraded by barrel tumbling in the conventional manner in an abrasive medium that includes aluminum oxide and silicon rock welted with a basar-water solution. Alternatively, the inserts may be grit blasted while set on end in a rubber mat to hasten abrasion. This abrasion is followed by tumbling in an abrasive medium to provide the desired smooth finish. The polygon shape prevents turning and movement of the insert, while the treatment with abrasives rounds the corner surfaces to prevent scraping of the wall of a retaining hole.